Magnetic colloidal fluids are essentially made of a solid magnetic material throughly dispersed in a liquid carrier. The magnetic material, usually magnetite (Fe.sub.3 O.sub.4) in extremely divided form is submicron in particle size produced by chemical precipitation, and the particles are subsequently coated with a layer of a surfactant retaining material, the coated particles are then permanently suspended in a carrier vehicle such as water mixed with a desired cationic or non-ionic surfactant. The resulting mixture is a colloidal magnetic fluid having several uses one of which is as a magnetic ink. A dispersing agent is usually included in the colloidal magnetic fluid formulation to prevent aggregation of the magnetic particles which could otherwise lead to flocculation and separation of the particles from the fluid.
In the development of the art of such magnetic colloidal fluids a limitation has been encountered in that the properties of the surfactant retaining coating which is usually a fatty acid such as oleate operates to limit flexibility in the properties of the surfactant so that particles receiving a certain coating are then useable only to produce a fluid with a certain charge characteristic.
Therefore, it is a general object of this invention to provide a technique of coating magnetic particles so that the properties of a resulting colloidal magnetic fluid containing them can be changed, by changing the net electrical charge applied to the magnetic particles.
Another object of this invention is to provide an aqueous based magnetic ink, by dispersing the so-coated Fe.sub.3 O.sub.4 into exclusively non-ionic surfactants, giving a non-ionic ink, in which particles have no net electrical charge.
A further object of this invention is to form a magnetic ink in which the so coated Fe.sub.3 O.sub.4 is dispersed in an anionic surfactant yielding an anionic ink, in which the particles have a net negative charge.
Another object is to form a magnetic ink by dispersing the so-coated Fe.sub.3 O.sub.4 in a mixture of cationic and non-anionic surfactants.